A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction between oxygen and hydrogen included in a hydrocarbon-based material such as methanol, ethanol, natural gas, or the like.
A fuel cell may be categorized as a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type according to the kind of electrolyte used. These fuel cells operate on the same general principles, but differ from one another according to the kind of fuel used, the operating temperature, the catalyst used, and the electrolyte used.
Recently, polymer electrolyte membrane fuel cells (PEMFCs) have been developed. They have excellent power output characteristics, low operating temperatures, and quick start and response characteristics compared to conventional fuel cells. Because of this, PEMFCs have a wide range of applications. Examples include mobile power sources for automobiles, distributed power sources for houses and public buildings, and small electric sources for electronic devices. A PEMFC system is essentially composed of a stack, a reformer, a fuel tank, and a fuel pump. The fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas which is supplied to the stack where it is electrochemically reacted with oxygen to generate electrical energy.
Another type of fuel cell is a direct methanol fuel cell (DMFC), in which liquid methanol fuel is directly introduced to the stack. The DMFC might not include a reformer, which is essential for the polymer electrolyte fuel cell.
According to the fuel cell system described above, the stack in a fuel cell system generates electricity and has a stacked structure including several to tens of unit cells stacked therein. Each unit cell is composed of a membrane-electrode assembly (MEA) and two separators (or bipolar plates).
The MEA includes a polymer electrolyte membrane interposed between an anode (referred to as a fuel electrode or oxidation electrode) and a cathode (referred to as an air electrode or reduction electrode). The polymer electrolyte membrane includes a hydrogen ion conductive polymer.
The separators not only work as passageways for supplying the fuel required for the reaction to the anode and for supplying oxygen to the cathode, but also as conductors for serially connecting the anode and the cathode in the MEA. An electrochemical oxidation reaction of the fuel occurs at the anode, and an electrochemical reduction reaction of oxygen occurs at the cathode, thereby producing electricity, heat, and water due to the transfer of electrons generated during this process.
The polymer electrolyte membrane included in the MEA shows excellent proton conductivity when it includes a predetermined amount of moisture. A typical fuel cell is additionally equipped with a humidifier to maintain the polymer electrolyte membrane at a predetermined moisture level, and this makes the structure of the fuel cell complicated and keeps the fuel cell from being down-sized.